LED in series

[pablo_101]

I'm currently building a light for a school project and need some help as my knowledge of Electronics is limited.
I have 4 White LED's joined up in series and will connect a sliding Potentiometer to it.
1. I wanted to power by mains supply and so think I need a transformer to reduce the voltage/ampage. Would 12V be enough for 4 LED's? Also what kind of ampage would be suitable?
2. Would a 100k or 10k Potentiometer be better?

 

[DerStrom8]

Welcome to Electro-Tech, Pablo_101!

Most white LEDs run on 3-4 volts, so 12 volts may not be enough. However, most PSUs actually produce more than the rated voltage, so it may work. You will have to experiment a little. You would also want to rectify the output from the transformer using 1N400x diodes before connecting your LEDs. You also should have a current-limiting resistor to protect the LEDs when the potentiometer is turned to its lowest value. All of the values (voltage, current, resistance, etc.) depend on the ratings of your LEDs. If you can provide us with this information, we will be able to help you more.
Best regards,
Der Strom

 

[pablo_101]

Am I right in saying the 1N400x diodes reduces the amps to 1 and only allows flow in one direction? Would the resistor just be a normal one?
Would it be ok to run off of a higher voltage then required (18v) without damaging the LEDS, as it may be easier for me to use two 9v batteries. Also if I ran it using batteries would it need the diode thing or a resistor?
Sorry If the questions seem silly I know hardly anything about this kind of thing.

 

[DerStrom8]

No, the 1N400x (1N4001, 1N4007, etc.) diodes will NOT reduce the current, but will only allow current to flow in one direction (change the alternating current, or AC, from the wall to direct current, or DC). The transformer will determine how much current can be supplied. The current rating of the transformer will determine what value resistor you will need in order to protect your LEDs. There is no such thing as a "normal resistor"--they all have specific values, and you must choose the correct one for your application.
I believe it would be possible to use two 9-volt batteries, but again, you'll need to choose the correct resistor. If you use batteries, you will not need the diodes, as the current is already DC. You will need the resistor, though.
Your questions are fine--Everyone starts somewhere
Good luck!
Der Strom

 

[ljcox]

Do you have a data sheet for the LEDs?

If so, post a copy so we can advise you further.

 

[pablo_101]

If the transformers output is already DC would there be a need for the diode?
By normal resistor I meant one that isn't special like LDP, It was just that you said current-limiting resistor and I didn't know if that was just the long name for a resistor or a special type.

Also I don't know a lot about the LED's I'm using as I just picked them out of a tray at school, however I was considering buying special superbright LED's as the 4 atm don't give enough light and I don't have room to add more LED's.

This may sound weird but what I'm doing is putting LED's inside an old style filament bulb (this is whythe room is limited) as the LED's are inside the bulb and the light fills the whole bulb does viewing angle matter?
Or is the mcd value the only thing that I should take into account when deciding which LED's are brighter?

 

[DerStrom8]

If the transformers output is already DC would there be a need for the diode?
By normal resistor I meant one that isn't special like LDP, It was just that you said current-limiting resistor and I didn't know if that was just the long name for a resistor or a special type.

Also I don't know a lot about the LED's I'm using as I just picked them out of a tray at school, however I was considering buying special superbright LED's as the 4 atm don't give enough light and I don't have room to add more LED's.

This may sound weird but what I'm doing is putting LED's inside an old style filament bulb (this is whythe room is limited) as the LED's are inside the bulb and the light fills the whole bulb does viewing angle matter?
Or is the mcd value the only thing that I should take into account when deciding which LED's are brighter?

If the output of the transformer is already DC, you would not need diodes. However, you must be SURE that the output IS DC. Do you mean a transformer like a wall-wart? (like one)
The "current limiting" only describes what the resistor is used for. In this case, it is used to limit the current to the LEDs so it doesn't burn them out. So yes, the resistor you want is a "normal resistor".

What do you know about your LEDs? Is there a way you can find a datasheet? You said you picked them out of a tray at school--would your teacher know? And what do you mean by "super bright"? Do you mean high-power, or just LEDs labeled "super bright"?

I also hope you don't plan to fit a transformer inside the bulb. Were you hoping to just screw the light bulb into an ordinary socket? As for the viewing angle, depending on your bulb, it may already diffuse the light throughout the bulb, so as long as you don't have all of your LEDs focused on one spot, it should work okay. The mcd value is the specific measure of the brightness of the LEDs--The higher the value, the brighter the LED.
Der Strom

 

[ljcox]

If you can't find a data sheet for them, connect a 220 Ohm resistor in series with the string of 4.

If there is no glow or if it is too dim, remove one of the LEDs.